The goal of this proposal is to develop a prototype intraoperative gamma- ray camera (IOGC) specifically designed for imaging gamma-emitting radioactive tracers incorporated into brain tumors in the intraoperative surgical environment. This proposal is motivated by the need for an instrument capable of dynamically identifying tumor cells so that they can be removed from a patient with a cerebral neoplasm without removal of excessive normal tissue. The gamma camera will be used in the surgical scenario where the patient is injected with a tumor seeking radiopharmaceutical before surgery, the surgical procedure to remove the tumor mass is performed, and the site is then imaged using the IOGC probe to locate residual subclinical tumor cells. The IOGC would be employed for radioisotopes such as 99mTc, 2O1Tl, and 123I which have low energy X- and gamma-rays. The intraoperative gamma-ray camera will use a small and light-weight detachable head comprising a mercuric iodide (HgI2) detector array. The use of HgI2 detector arrays in this application will make it possible to construct an imaging head that is very compact and has high signal to noise ratio. The detector head will be optimized for operation proximal to the tissue of the patient in the surgical field in order to give the best sensitivity, spatial resolution, and contrast. The detector head will be mounted on a counterbalanced arm that can be positioned easily during surgery and will be connected via cable to the rest of the camera. During the first phase of the work a study will be conducted to determine the optimal construction of the front end and the resulting design will be incorporated into a 4 x 4 (16 pixel) prototype array which will be fabricated and evaluated.